Administrator Guide

State Description
If that transition is not successful, BGP resets the ConnectRetry timer and transitions to the Active state when
the timer expires.
Active The router resets the ConnectRetry timer to zero and returns to the Connect state.
OpenSent After successful OpenSent transition, the router sends an Open message and waits for one in return.
OpenConfirm After the Open message parameters are agreed between peers, the neighbor relation is established and is in the
OpenConfirm state. This is when the router receives and checks for agreement on the parameters of open
messages to establish a session.
Established Keepalive messages are exchanged next, and after successful receipt, the router is placed in the Established
state. Keepalive messages continue to be sent at regular periods (established by the Keepalive timer) to verify
connections.
After the connection is established, the router can now send/receive Keepalive, Update, and Notification messages to/from its peer.
Peer Groups
Peer groups are neighbors grouped according to common routing policies. They enable easier system configuration and management by
allowing groups of routers to share and inherit policies.
Peer groups also aid in convergence speed. When a BGP process needs to send the same information to a large number of peers, the
BGP process needs to set up a long output queue to get that information to all the proper peers. If the peers are members of a peer group
however, the information can be sent to one place and then passed onto the peers within the group.
BGP Attributes for selecting Best Path
Routes learned using BGP have associated properties that are used to determine the best route to a destination when multiple paths exist
to a particular destination.
These properties are referred to as BGP attributes, and an understanding of how BGP attributes influence route selection is required for
the design of robust networks. This section describes the attributes that BGP uses in the route selection process:
• Weight
• Local Preference
• Multi-Exit Discriminators (MEDs)
• Origin
• AS Path
• Next Hop
NOTE:
There are no hard coded limits on the number of attributes that are supported in the BGP. Taking into account
other constraints such as the Packet Size, maximum number of attributes are supported in BGP.
Communities
BGP communities are sets of routes with one or more common attributes. Communities are a way to assign common attributes to multiple
routes at the same time.
NOTE: Duplicate communities are not rejected.
Best Path Selection Criteria
Paths for active routes are grouped in ascending order according to their neighboring external AS number (BGP best path selection is
deterministic by default, which means the bgp non-deterministic-med command is NOT applied).
The best path in each group is selected based on specific criteria. Only one “best path” is selected at a time. If any of the criteria results in
more than one path, BGP moves on to the next option in the list. For example, two paths may have the same weights, but different local
preferences. BGP sees that the Weight criteria results in two potential “best paths” and moves to local preference to reduce the options.
If a number of best paths is determined, this selection criteria is applied to group’s best to determine the ultimate best path.
In non-deterministic mode (the bgp non-deterministic-med command is applied), paths are compared in the order in which they
arrive. This method can lead to Dell EMC Networking OS choosing different best paths from a set of paths, depending on the order in
which they were received from the neighbors because MED may or may not get compared between the adjacent paths. In deterministic
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Border Gateway Protocol (BGP)